1. Field of the Invention
This invention pertains to a communications controller in a data processing system and more particularly to apparatus for synchronizing a stream of data bits transmitted from the communications controller to a device in a byte control protocol mode with a byte timing signal generated in the device.
2. Description of the Prior Art
Highly flexible and cost effective communication line adapters have been provided for coupling data processing systems to communication channels such as those associated with a publicly accessible, e.g., telephone, communications network. Such systems have been embodied in hardware/firmware architectures which respond to commands from a communication processor associated with a central processing unit and enter into either a receive mode, a transmit mode, or concurrent transmit/receive modes for transferring data messages between the communication processor and the communication channel. Data transfers occur under the control of a firmware system acting in concert with a microprocessor within the adapter to assemble and disassemble whole or partial data bytes of varying bit sizes. System architectures which have been used readily accommodate an expansion of capacity and exhibit dynamic flexibility.
Communication line adapter systems are required to interface with a synchronous communications network such as, for example, has been specified by the International Telegraph and Telephone Consultative Committee (CCITT) and described in the "CCITT Sixth Plenary Assembly--Orange Book, Vol. VIII.2 Public Data Networks", published in 1977 by the International Telecommunication Union, Geneva. Various specifications, in particular those designated X.21, X.24 and X.27, promulgated by the CCITT call for generation within the data network of bit and byte timing signals which are used for establishing a byte control protocol (BCP) or a bit oriented protocol (BOP) for organizing and handling data messages.
U.S. application Ser. No. 053,110 which was abandoned May 29, 1981 entitled, "Communication Line Adapter For a Bit and Byte Synchronized Data Network" describes a communication adapter that supports the BCP mode of communication.
In the BCP mode the device generates a byte timing signal and a sequence of bit timing signals. The transmission of data bits from the adapter to the device is synchronized with the byte timing signal in a microprocessor in the adapter by means of a firmware routine stored in a memory in the adapter. In addition, hardware in the adapter under control of the microprocessor assembles the data bits of the byte and transmits the data bits to the device synchronized to the byte timing signal.
However, present day communications subsystems have a requirement to operate with many types of communications devices such as dialing units, MODEMs, touch tone receivers, cathode ray tube displays, remote printers, etc. The prior art adapters require modification for communicating with the various devices, each of which has a different mode of control.
Accordingly, a communications controller having a common logic section for handling all kinds of communications devices and individual adapter boards for controlling each device was required. This made the byte synchronization approach of the prior art systems unwieldy. The microprocessor in the common logic section would have difficulty in keeping up with the throughput requirements of the multitude of devices. A microprocessor with its associated logic into the individual adapter board would be inordinately expensive to design, manufacture and support in the field.